1. Field of the Invention
This invention relates generally to a method and an apparatus for coating the interior surface of a pipe and more particularly to a method and apparatus for providing a uniform corrosion resistant protective coal tar epoxy coating on the interior surface of a pipe subject to a high corrosive environment.
2. Description of the Prior Art
To protect metal pipe from corrosion due to water, steam, chemicals, acids, and so forth, it is required to coat the inner surface of the pipe with a continuous coating which is not only free of holidays, pin holes and the like, but one which is corrosion resistant and which will protect the pipe against attack from the corrosives contained in the materials being handled. Sewer pipes are particularly vulnerable to attack by corrosive agents because of their exposure to various chemicals, acids and gases such as, for example, H.sub.2 S.
Some of the various patented practices developed over the years for coating the interior surfaces of a pipe include the following:
U.S. Pat. No. 2,353,951--Wood et al. This patent relates to the deposit of a suspension of colloidal graphite into the pipe and thereafter inserting a roller member in intimate contact with the interior surface and rotating the roller member so as to cause the graphite particles to penetrate and fill the interstices in the surface. U.S. Pat. No. 2,845,366--Schroeder. This patent relates to metallizing very long pipe lengths with aluminum while the pipe is rotated, but maintained longitudinally stationary, or vice versa, and fusing the aluminum coating into the roughened wall. U.S. Pat. No. 3,359,943--Briggs et al. This patent relates to the application of a zinc silicate coating on the inner surface of a long length pipe and employs a flexible conduit and nozzle with centering means which is drawn through the pipe as the coating is sprayed on the interior surface. U.S. Pat. No. 3,974,306--Inamuro et al. This patent discloses a method wherein the coating composition is applied to the interior surface of a preheated pipe by feeding a dry powder at one end and sucking air at same time from the other end.
While the above patents illustrate several known methods of coating interior surfaces of pipe, they are not intended to suggest that these are the only methods. Methods will necessarily vary depending on the materials employed, the nature of the pipe and the use to which it will be subjected. Other examples of known methods of coating the interior surface of a pipe can be found in the following U.S. patents: U.S. Pat. No. 2,399,606--Schuh et al U.S. Pat. No. 2,731,690--Coupland U.S. Pat. No. 2,563,843--Johnston U.S. Pat. No. 2,729,190--Pawlyk U.S. Pat. No. 3,007,810--Hobrock U.S. Pat. No. 3,347,699--Hitzman U.S. Pat. No. 3,563,791--Janci U.S. Pat. No. 4,254,165--Phelps
Known methods of coating the interior cylindrical surface of pipe generally suffer from one or more disadvantages. For example, the lamination bond may be weak or production costs may be undesirably high. To increase bonding, resort is often made to special materials which are costly and/or require special pipe treating procedures such as preheating.
Spray coating is a popular method of applying coatings, but generally requires relative rotation between the spray head and pipe, with the movement causing a spiral effect in the coating. Unless the movement and spray pattern are accurately controlled, gaps may occur. To compensate for gaps, coating is often applied in multiple layers undesirably increasing production time and also increasing the possibility of uneven lapping. Spray coating is generally limited to low viscosity materials because of the tendency of viscous materials to clog small orifices.
Other known methods employed for coating long length pipe, i.e. a pipe whose length is several times its diameter, in the neighborhood of eighteen or twenty feet, suggest applying the coating from opposite ends. This causes an overlap at the center of the pipe, and because the first applied coating may set before the other coating is applied, such coatings are prone to delamination.